Present invention relates to lasers and, more particularly, to a slab laser resonator design of improved beam quality.
A slab geometry for an active (either solid-state or gas) lasing medium allows for active medium volume scaling by increasing the area of the slab of constant thickness, which is necessary to provide efficient cooling of the slab by heat conduction.
Diffusion-cooled, slab gas CO2 lasers were first introduced by Gabai, Hertzberg and Yatsiv in Paper TUB4 at CLEO""84, Anaheim, Calif., U.S.A, May 1984. Practical applications of this design are limited since its two-mirror stable resonator does not allow for efficient low order mode selection in the plane of the slab. Additionally, relatively large interelectrode gap (4.5 mm) in this design results in relatively low waveguide losses, leading to insufficient high-order mode discrimination in the transverse direction (perpendicular to the plane of the slab). The slab CO2 laser concept is extended to cover small gap slab resonators in U.S. Pat. No. 4,719,639 to Tulip (the ""639 patent). Waveguide losses in the small gap slab laser are significant enough in this design to discriminate against high order transverse modes. Additionally, an off-axis unstable confocal resonator in the ""639 patent (see also Siegman, Lasers, University Science Books, Mill Valley, Calif., 1986, pp. 903-904) allows for efficient selection of a low order mode in the plane of the slab. However, a disadvantage of an unstable confocal resonator is a non-gaussian output beam profile in the plane of the slab (Hodgson and Weber, Optical Resonators, Springer, 1997, p. 507). Another disadvantage is that the output laser beam is of a high-aspect-ratio rectangular shape, which leads to the necessity for using expensive beam reshaping optical devices.
Another prior art slab laser was described in U.S. Pat. No. 5,353,297 to Koop et. al. This slab laser has a folded unstable resonator and, similarly to other unstable confocal resonators, has a disadvantage of a non-gaussian laser beam profile in the plane of the slab. Another disadvantage is that the output laser beam is of a rectangular shape, which leads to the necessity for using expensive beam reshaping optical devices.
Another prior art slab laser was described in U.S. Pat. No. 5,892,782 to Vitruk et. al. This design improves the quality of the laser beam in the direction perpendicular to the plane of the slab in a large gap slab resonator. It is achieved by introducing additional mode discrimination mechanismsxe2x80x94the slab is bent and is of a variable thickness and the electrode surfaces include step-like discontinuities in the middle of the resonator. Disadvantages of this prior art laser include a non-gaussian output beam profile in the plane of the slab as well as a rectangular output laser beam since it utilizes a two-mirror confocal unstable resonator.
A multi-fold stable resonator can be used with a slab shaped active media of either solid-state or gas lasers. A significant advantage of this resonator is the symmetric gaussian shape of the output laser beam. A further advantage of the multi-fold resonator used in CO2 gas lasers is their large resonator length, which helps prevent line-hopping and helps stabilize the output laser power.
A slab laser design with multi-fold resonator was devised in U.S. Pat. No. 5,661,746 to Sukhman et. al. (the ""746 patent). This design utilizes a large gap slab design and uses a three-mirror, multi-fold, stable, free-space non-telescopic resonator. The output coupler as well as one of the folding mirrors is flat, while the other mirror is concave. Transverse mode size in this free-space design is defined by the stable mirror configuration and is chosen to be smaller than the slab thickness (interelectrode gap). An unwanted parasitic oscillation between the folding mirrors is eliminated by significant tilt between the folding mirrors. A disadvantage of this design is that the number of passes through the resonator is limited to a very few, which limits the output laser power to about 100 W for practical devices. Since the greater number of passes in this design requires a smaller tilt between the folding mirrors, this causes an unwanted laser oscillation between the two large aperture folding mirrors.
Many disadvantages of the traditional multi-fold resonators can be avoided if an optically unstable folding mirror configuration is used (e.g. a folding mirror resonator axis positioned outside of lasing medium, as in the ""746 patent), which eliminates an unwanted laser oscillation between the folding mirrors. Furthermore, if folding mirrors form a telescopic pair, as described by Yu. A. Ananev in xe2x80x9cOptical resonators and problem of divergence of the laser radiationxe2x80x9d (in Russian 1979, Nauka, Moscow, p. 147, see also O. B. Danilov et. al., xe2x80x9cInvestigation of the misalignment of a resonator with a telescopic angular selector in a photodissociation laserxe2x80x9d, Sov. J. Quant. Electr., vol. 6, No. 1, pp. 109-112), then a large width laser beam could cover a large aperture active lasing medium. At the same time, the Fresnel number of such resonator would remain small enough to allow for easy low order free-space mode selection. Such a prior art telescopic laser resonator is presented schematically in FIG. 1A. Laser 10 consists of a large aperture active lasing medium 11, a concave folding mirror 12, a convex folding mirror 13 and two resonator mirrors 14 and 15. An intracavity laser beam 16 covers a large aperture lasing medium, while a large width output beam 17 lowers the power load on the resonator mirror 15, which also acts as an output coupler. FIG. 1B shows schematically a similar multi-fold telescopic resonator, which is applied to large area slab waveguide gas laser in German patent DE 19609851 issued to Anikitchev. Unlike the free-space resonator in FIG. 1A, the active lasing medium 16 in FIG. 1B is slab-shaped and is confined in the gap between the two planar electrodes. Small inter-electrode separation makes this resonator of a hybrid type: it is a waveguide in the direction perpendicular to the plane of the slab and it remains stable free-space in the plane of the slab. Advantages of this resonator include large mode volume and a gaussian beam profile in the plane of the slab as well as efficient lowest order waveguide mode selection in the direction perpendicular to the plane of the slab. A disadvantage of such a resonator is that it is not suitable for larger gap slabs since the waveguide mode selection mechanism in the direction transverse to the plane of the slab is not effective enough to eliminate higher order modes. Consequently, the use of this design is limited to very thin slabs leading to a rectangular output beam shape of high aspect ratio. Additionally, large radius folding mirrors used in German patent DE 19609851 make this resonator very sensitive to the misalignment.
The present invention is aimed to achieve a high quality, nearly-gaussian, symmetrical non-rectangular output laser beam from a multi-fold slab laser resonator with substantially reduced sensitivity to misalignment. A second object of the present invention is to provide a nearly symmetrical output laser beam from a large thickness slab laser with multi-fold telescopic resonator. Another object of the present invention is to improve the beam quality in the direction perpendicular to the plane of the slab of a resonator employing a large thickness slab and for which a waveguide mode selection mechanism is not effective to discriminate against high order modes. Furthermore, it is another object of the present invention to simplify a four-mirror, multi-fold telescopic design. Finally, it is also a object of the current invention to provide for a multi-fold telescopic resonator design suitable for a very large area slab lasing media.
The slab laser with stable multi-fold telescopic resonator according to present invention consists of a first and second elongated resonator walls having a light reflecting surfaces, a slab of lasing between said elongated walls, a means for exciting the said lasing medium, a first and second folding mirrors and at least one resonator mirror. Folding mirrors are positioned on the opposite ends of the slab and form either confocal or near-confocal unstable pair, between which a laser oscillation is inhibited by the shape and angular alignment of the mirrors. A gaussian output beam profile in the plane of the slab is achieved through a free-space, low Fresnel number, multi-fold stable resonator design in which the folding mirrors provide significantly expanded width of the intracavity laser beam as it propagates through the resonator. An intracavity laser beam is confined between/by the slab boundaries, as it propagates through the slab lasing medium.
A reduced sensitivity to misalignment is achieved through the use of two short-radii concave folding mirrors forming a nearly-confocal or confocal optical resonator of negative branch.
A nearly-symmetrical output beam profile is achieved by using a large thickness slab, so that the width of the beam could be matched to its height, while the resonator includes a slit diaphragm(s) with an aperture slightly smaller than the thickness of the slab. Such a diaphragm is positioned between the slab lasing medium and resonator mirrors and is aimed to substitute the waveguide mode selection mechanism and to eliminate the higher order laser modes in the direction transverse to the plane of the slab. Slit diaphragm could be manufactured as a part of the discharge electrodes of the gas slab laser. Slit diaphragm could be used with either the positive- or negative-branch folding mirror configuration.
Additionally, a three mirror multi-fold telescopic resonator is provided by substituting one of the resonator mirrors with a surface of one of the folding mirrors. Three mirror design is characterized with simplicity of manufacture and alignment and could be used with both the positive- and negative-branch folding mirror configuration.
Finally, a four-mirror multi-fold telescopic resonator is provided to cover a very large area slab lasing media. This design is characterized by the on-axis confocal or near-confocal folding mirror alignment and could be used with both the positive- and negative-branch folding mirror configuration.